Semiconductor device and fabrication method

ABSTRACT

A thin-film semiconductor device comprising at least a semiconductor element and a wiring is disclosed. A thin film of a protective insulating material is formed on the lower surface of the semiconductor element, and a substrate is bonded on the lower surface of the thin film. A method for fabricating the thin-film semiconductor device is also disclosed, in which a thin-film semiconductor circuit is formed on a silicon-on-insulator wafer, the silicon substrate on the reverse side of the silicon-on-insulator wafer is etched off, a thin-film semiconductor chip is formed and attached to the substrate, and the thin-film semiconductor chip and the substrate are wired to each other by printing.

This is a continuation of application Ser. No. 08/280,935, filed Jul.27, 1994.

BACKGROUND OF THE INVENTION

The present invention relates to a reliable and low-cost semiconductordevice, or more in particular to an IC (Integrated Circuit) card or amultichip module.

Conventional techniques for IC cards are described in "InformationProcessing Handbook", first edition, pp. 302-304, compiled byInformation Processing Society of Japan and published by Ohm, May 30,1989. The same reference contains at pp. 242-244 also the description oftechniques for packaging the IC card. The structure of IC cards isdescribed in "IC Cards", first edition, p. 33, compiled by The Instituteof Electronics, Information and Communication Engineers and published byOhm, May 25, 1990. Also, an IC card using a thin LSI is disclosed inJP-A-3-87299, Apr. 12, 1991.

FIGS. 1, 2 and 3 are sectional views showing configurations of IC cards.

In FIG. 1 showing a conventional IC card configuration, a chip 211 isbonded to a portion having a contact 210, connected to a printed board212 by a bonding wire 216, and sealed by resin 215. This module isembedded in a center core 213 of a hard material. The card surface iscovered with an oversheet 209 and an oversheet 214.

FIG. 2 shows another example of the prior art.

A semiconductor chip is bonded to a substrate 207 by a adhesive agent207a. Due to a thick silicon substrate 217, however, the semiconductoris connected by being bonded to the substrate 207 by the adhesive 207awhile absorbing the unevenness through a bonding wire 218.

In the example shown in FIG. 3, an IC 6 has a great thickness of about200 to 400 μm. This bulk IC 6 is bonded to a card board 8 by an adhesive7. Since the bulk IC is thick, however, the uneven wiring patterns onthe IC and a substrate wiring 10 are connected by a wire bonding 9. Inthis case, the bulk IC is easily subjected to bending stress andtherefore stress relaxation is required. Also, in view of the limitedsizes of the bulk IC, the structural requirement for improving thebending strength and the difficulty of reducing the number ofwire-adhesive steps, the cost tends to increase.

JP-A-3-87299 (Apr. 12, 1991) has rendered well known an IC cardconfiguration in which an IC module having a very thin LSI ground verythin leaving active elements is fitted in a surface recess.

This conventional configuration is shown in FIG. 4. A semiconductorelement 204 is bonded on a substrate 207 by an adhesive 207a. A wiring208 for connecting semiconductor elements is connected to a conductivepad 202 by way of a through hole 203. This conductive pad 202 is furtherconnected to the wiring on the substrate 207 by conductive paste 201.

The problem of this configuration, as shown in FIG. 4, is that anadhesive layer is in direct contact with the lower surface of thesemiconductor element 204 such as a transistor and ionic contaminantseasily intrude the semiconductor element, thereby extremelydeteriorating the reliability. FIG. 5 is a diagram showing a problemspecific to an IC card configured using a thin LSI disclosed inJP-A-3-87299 (Apr. 12, 1991). A thin LSI 41 mounted on a thick cardsubstrate 42 is subjected to tensile or compressive stress on the frontand reverse surfaces when the card substrate 42 is bent, therebyexerting a large stress on the LSI chip. The resulting thin structureand low mechanical strength under a large stress causes the IC to beeasily broken by the stress. This gives a rise to a new problem of aconsiderably reduced reliability.

As described above, the IC card using a thin LSI layer including a thinsemiconductor element is easily affected by ionic contaminants. Also,the thinness leads to a low mechanical strength. In the conventional ICcards using a bulk LSI, a bulk IC chip is attached on a thin,easy-to-bend card and wire-bonded. Therefore, the IC is easily brokenand is low in reliability. Further, the increased number of packagingsteps makes a cost reduction difficult.

SUMMARY OF THE INVENTION

The object of the present invention is to solve the above-mentionedproblems of the prior art and to provide a semiconductor circuit, ormore in particular a IC card or multichip module high in reliability andlow in cost.

According to one aspect of the invention for solving the above-mentionedproblems, there is provided a thin-film semiconductor device comprisingat least a semiconductor element and a wiring, wherein a thin film of aprotective insulating material for protecting the semiconductor elementis formed on the lower surface thereof in contact with the semiconductorelement, and the surface of the protective insulating film is bonded toother substrate.

According to another aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin-film semiconductor circuit includes a thin film of asemiconductor circuit formed on a silicon-on-insulator (hereinafterreferred to as SOI) wafer, another substrate coupling the thin-filmsemiconductor circuit on the opposite side formed with the semiconductorcircuit, and a hardenable conductive material for connecting the wiringprepared on the substrate and the wiring of the thin-film semiconductorcircuit.

According to a third aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin-film semiconductor circuit is taken out of the mainsurface of the SOI wafer with the insulator layer inward thereof as aboundary.

According to another aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin-film semiconductor circuit and the other substrate arebonded by a rubber-like adhesive.

According to another aspect of the invention, there is provided asemiconductor device wherein the main surface is bonded to anothersupport substrate, and then the SOI wafer substrate is ground or etchedoff.

According to still another aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the other substrate for coupling the thin-film semiconductorcircuit is of a flexible card shape.

According to a further aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the other support substrate is flexible.

According to a still further aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin-film semiconductor circuit and the support substrateare bonded to each other by an adhesive separable under ultraviolet ray.

According to an even further aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the wiring by a liquid conductive material is a printed wiringwith a rotary drum.

According to another aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin-film semiconductor circuit is located at the center ofthe same depth from the front and reverse surfaces of the othersubstrate.

According to still another aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin-film semiconductor circuit is bonded to one substrateand covered by being bonded to the other substrate of the samethickness.

According to a further aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin-film semiconductor circuit is formed with a wafer otherthan SOI.

According to a still further aspect of the invention for solving theabove-mentioned problems, there is provided a card-like semiconductordevice with a thin IC chip built in the card, wherein the thickness ofthe IC chip is 110 microns or less for the completed card thickness of760 microns or more, 19 microns or less for the completed card thicknessof 500 microns or more, and 4 microns or less for the completed cardthickness of 250 microns or more.

According to an even further aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thickness of the IC chip is at least 4 microns or less forthe completed IC card thickness 250 microns or less.

According to another aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin IC chip is located at the center along the thickness ofthe card.

According to still another aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin IC chip is held between two or more card substrates.

According to a further aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the thin IC chip is preferably bonded in such a configuration asto be held between card substrates by a flexible adhesive.

According to a still further aspect of the invention for solving theabove-mentioned problems, there is provided a semiconductor devicewherein the protective insulating material is silicon nitride.

The above-mentioned object can be achieved by providing a semiconductordevice in which a semiconductor circuit prepared from asilicon-on-insulator (hereinafter referred to as SOI) wafer in thin filmform is coupled to other substrate, to which a wiring prepared inadvance and a wiring for the above-mentioned semiconductor circuit areconnected by a liquid conductive material, and this liquid conductivematerial is then hardened.

This thin-film semiconductor circuit can be coupled to the othersubstrate after being taken out of the main surface with the insulatorlayer inward of the SOI wafer as a boundary. Specifically, the thin-filmsemiconductor circuit can be prepared by coupling the main surface ofthe semiconductor circuit formed of the SOI wafer to another supportsubstrate and then by grinding or etching off the substrate of the SOIwafer.

The thin-film semiconductor circuit is preferably coupled to the othersubstrate by a rubber-like adhesive.

Also, the other substrate to which the thin-film semiconductor circuitis coupled preferably is card-shaped and flexible.

The other support substrate is also preferably flexible.

The thin-film semiconductor circuit and the other support substrate arebonded to each other by the use of an adhesive (hereinafter referred toas the ultraviolet-separating adhesive) the bonding strength of whichdecreases under ultraviolet ray, thereby facilitating separation of theother support substrate during the process.

Further, the liquid conductive material is effectively applied on thewiring by printing using a rotary drum.

Furthermore, the IC card can be easily fabricated by providing thethin-film semiconductor circuit placed at the center of the same depthfrom the front and reverse surfaces of the substrate, or morespecifically, by bonding the thin-film semiconductor circuit to onesubstrate and then covering it by bonding with another substrate of thesame thickness.

Although the foregoing description refers to the case of using athin-film semiconductor circuit prepared on an SOI wafer, a similareffect of course is obtained by the use of a thin-film semiconductorcircuit formed on a wafer other than SOI.

The above-mentioned method for attaching a protective insulating layeron the reverse surface of a thin semiconductor element causes theprotective insulating film to prevent intrusion of ionic contaminantsfrom the reverse side of the semiconductor element nearest to theexternal environment, and therefore the reliability is improved. As aresult, an IC card with an improved durability can be fabricated evenwhen a thin LSI is bonded to a substrate using a low-cost organicadhesive generally containing considerable ionic impurities.

The use of silicon nitride as the protective insulating film with alarge thermal expansion coefficient suppresses the curl of the thin LSIfilm due to the internal residual stress, thereby contributing to animproved reliability of the IC card.

When an SOI wafer is used, the inner insulator layer acts as a stopperin processing, so that a thin film IC can be prepared uniformly with ahigh reproducibility. The thin-film IC thus prepared is 5 to 10 μm inthickness. This much thin IC is resistive to bend, and when bonded witha flexible adhesive to a thin substrate like an IC card, exhibits a highbending strength leading to a high reliability.

Also, the thin-film IC, which itself tends to break easily, ispreferably mounted on a support substrate in advance to add stability.The support substrate to which the IC is coupled can be reliably removedat low temperatures by using an adhesive separable under ultravioletray. The thin-film IC attached to the card is so thin that wiring ispossible with a conductive paste between the substrate and the IC. Ascompared with the conventional wire-bonding method using a gold wire,the method according to the invention is suited to mass production offlat, thin IC cards with low material cost.

The above-mentioned methods are applicable not only to the IC card butalso to similar packaging of ICs as well as to multichip packaging.

Consider the section of a bent portion of a flat IC card. The curvedsurface develops an elongation and the reverse side thereof acompression. Under this condition, the central part of the section ofthe IC card is under a small stress free of compression. If a thin ICchip exists at this portion, such an IC chip is subjected to lessstress. The IC chip is preferably thin. In the case where the card isthick, however, the card rigidity increases the critical curvature,thereby making it harder to bend. For this reason, the IC chip can bethick to some degree. In the case where the IC card is thin and easy tobend, on the other hand, the IC chip must also be reduced in thicknessin order to relax the stress on the IC chip. In preparing a thin IC, thethinner the IC, the higher accuracy is required of fabrication devices.Changing the required IC chip thickness in accordance with the thicknessof the IC card, therefore, is very important from the economic viewpointand also for securing the required reliability. In this way, thecorrelationship between the thicknesses of the IC card and the IC chipis recognized to produce an economic and reliable IC card. Specifically,the thickness of the IC chip is rendered 110 microns or less for a cardin completed form having the thickness of 760 microns or more, 19microns or less for a card in completed form having the thickness of 500microns or more, and 4 microns or less for a card in completed formhaving the thickness of 250 microns or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the essential parts of a conventionalIC card for explaining the present invention.

FIG. 2 is a sectional view showing the essential parts of a conventionalIC card for explaining the present invention.

FIG. 3 is a sectional view showing the configuration of the essentialparts of a conventional IC card.

FIG. 4 shows the essential parts of a conventional IC card forexplaining the present invention.

FIG. 5 is a sectional view showing the configuration of a conventionalIC card bent with a thick substrate.

FIG. 6 is a sectional view showing the essential parts of asemiconductor device according to an embodiment of the invention.

FIGS. 7A, 7B, 7C are sectional views showing the steps of fabricatingthe semiconductor device of FIG. 6.

FIG. 8 is a sectional view showing the essential parts of asemiconductor device according to still another embodiment of theinvention.

FIG. 9 is a sectional view showing the configuration of an embodiment ofthe invention.

FIG. 10 is a diagram showing the steps of fabricating an IC cardproviding a semiconductor device according to an embodiment of theinvention.

FIG. 11 is a sectional view showing a thin-film IC fabricated from a SOIwafer.

FIG. 12 is a sectional view showing a thin-film IC with a supportsubstrate coupled to a card substrate.

FIG. 13 is a sectional view showing a thin-film IC with the supportsubstrate removed by irradiation of ultraviolet ray.

FIG. 14 is a sectional view showing the condition in which a thin-filmIC and a substrate are connected with conductive ink.

FIG. 15 is a sectional view showing a configuration of the inventionused for a multichip module.

FIG. 16 is a sectional view of an apparatus for printing the wiring withconductive ink.

FIG. 17 is a sectional view showing a thin-film IC embedded in a cardsubstrate.

FIG. 18 is a sectional view for explaining the steps for producing theconfiguration shown in FIG. 10.

FIG. 19 is a sectional view showing the essential parts of an IC cardaccording to an embodiments of the invention.

FIG. 20 is a sectional view of the essential parts of an IC card forexplaining the principle of the invention.

FIGS. 21A, 21B are sectional views of the essential parts of an IC cardshowing the fabrication steps thereof according to an embodiment of theinvention.

FIGS. 22A, 22B are sectional views of the essential parts of an IC cardshowing the fabrication steps thereof according to another embodiment ofthe invention.

FIG. 23 is a plan view of an IC card according to still anotherembodiment of the invention.

FIG. 24 is a sectional view of an IC card according to a furtherembodiment of the invention.

FIG. 25 is a diagram showing the relation between LSI-card thicknessratio and LSI surface stress according to the invention.

FIG. 26 is a diagram showing the relation between radius of curvatureand LSI surface stress against IC card thickness according to theinvention.

FIG. 27 is a diagram showing the relation between LSI-card thicknessratio and LSI surface stress according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A semiconductor device having the configuration according to theinvention will be specifically described with reference to embodiments.A sectional view of an embodiment of the invention is shown in FIG. 6.

In FIG. 6, a conductive paste 201 is connected to a pad 202 electricallycoupled by way of a through hole 203 on a wiring 208, and thuselectrically connected to components external to the chip. The wiring208 connects semiconductor elements 204 thereby to form a circuit. Thesemiconductor elements 204 are bonded to a substrate 207 by an adhesive207a through silicon nitride 206. The silicon nitride 206 is coated onthe lower surface of silicon oxide 205.

The semiconductor element 204 has the lower surface thereof formed withthe silicon oxide 205 for electrical isolation of the semiconductorelement. The semiconductor element 204, which is prepared by use oflayers of silicon-on-insulator (SOI) wafer, is formed very thin. At thesame time, the silicon oxide 205 provides an inner insulator layer ofthe SOI wafer. The silicon oxide itself has no function of shutting offNa, H₂ or H₂ O, resulting in the semiconductor element beingcontaminated by ionic substances and a protracted operating instability.Especially when the distance between the lower surface of thesemiconductor element and the external environment approaches 10 micronsor less with a silicon oxide as a thin film therebetween, impuritiesdiffusion is included in the distance even at the LSI-operatingtemperatures, and a great problem is posed threatening the LSIreliability. With a thicker silicon oxide, by contrast, the wafer bendsat temperatures higher than 1000° C. in the wafer processing steps, andthe resulting misalignment or the like makes fine processing impossible.According to this embodiment, silicon nitride is attached after a thinLSI film is formed. The silicon nitride film has superior chemical,physical and electrical properties as a passivation film. Especially,its high imperviousness to Na, H₂ and H₂ O exhibits a superior effect ofprotecting the semiconductor elements of a thin LSI.

The thickness of the silicon nitride is, for example, one micron atmaximum as a limit of developing a crack and about 0.01 micron atminimum taking the Na-ion diffusion range into account for the maximumoperating temperature 90° C. of the IC card.

The steps for fabricating a semiconductor device in FIG. 6 are shown inFIGS. 7A, 7B, 7C. First, as shown in FIG. 7A, there is formed asemiconductor device comprising semiconductor elements 204, a wiring 208for connecting the semiconductor elements 204 to each other, and a pad202 on the wiring 208 each by way of a through hole 203, all of whichare formed on the main surface of a silicon-on-insulator having asilicon oxide 205 on a silicon substrate 217.

Next, as shown in FIG. 7B, the silicon substrate 217 is selectivelyetched off by potassium hydroxide or hydrazine which has such aselective-etching function. Further, as shown in FIG. 7C, siliconnitride is coated on the reverse side of the silicon oxide 205. Afterthat, as shown in FIG. 6, an LSI formed in thin film is bonded by theadhesive 207a and connected to the substrate using the conductive paste201, thereby completing a semiconductor device.

The thin-film LSI is reduced to the thickness of the order of 10microns, and therefore, after being bonded to the substrate, can beconnected to the substrate by paste or ink-like liquid material due to asmall unevenness with respect to the substrate. Consequently, a very lowand flat connection is made possible in a shape optimal to an IC card.Also, the conductive paste, which is as thin as about 10 microns andhigh in flexibility, is highly resistive to bending and thermalexpansion.

Another embodiment of the invention is shown in FIG. 8. In FIG. 8, aconductive paste 201 connected to a pad 202, which in turn iselectrically connected to a wiring 208 by way of a through hole 203, iselectrically connected to the environment external to the chip. Thewiring 208 connects semiconductor elements 204 to each other thereby toform a circuit. The semiconductor elements 204 are bonded to a substrate207 by an adhesive 207a through silicon nitride 206.

In the embodiment of FIG. 8, the silicon oxide film of FIG. 6 isreplaced by a silicon nitride film. As a means of reducing the thicknessof the LSI, though not restrictive, the lower surface of thesemiconductor element is protected by silicon nitride after reducing thethickness. This silicon nitride film has the effect of regulating thethermal expansion coefficient and prevents the thin LSI from beingcurled due to the internal residual stress.

FIG. 9 is a sectional view showing the configuration of a semiconductordevice according to an embodiment of the invention. A thin-film IC 1 isbonded to a card substrate 2 by means of an adhesive 3. The thickness ofthe thin-film IC, depending on the number of layers of wiring andtransistors employed, is in the range of about 5 to 10 μm. This thinnesspermits a substrate wiring 5 to be connected with the wiring pattern onthe thin-film IC 1 by conductive ink 4. The thin-film IC, unlike thebulk IC, is easily bent, and therefore suitably is bonded to a substrateof plastics such as a card substrate which is also easy to bend. Also, arubber-like or a flexible adhesive is preferably used for bonding thethin-film IC 1 and the card substrate 2. The rubber-like property andthe flexibility reduce the bending stress acting on the thin-film IC.

The steps of fabricating an IC card providing a semiconductor deviceaccording to an embodiment of the invention are shown in FIG. 10. First,a thin-film IC (LSI) is formed on a SOI wafer (step 101). Next, thesilicon substrate on the reverse side is etched off by potassiumhydroxide (step 102). In the process, the silicon oxide film inward ofthe SOI wafer cannot be removed by potassium hydroxide, and thereforethe thin-film IC can be fabricated in self-aligning mode (step 103).Also, since the thin film IC, if alone, would curl due to the internalstress, the main surface of the SOI wafer is bonded in advance to asupport substrate by adhesive. A sectional view of the SOI wafer withthe silicon substrate removed is shown in FIG. 11. Numeral 11 designatesthe support substrate, numeral 12 the adhesive, numeral 13 the thin-filmIC, and numeral 14 the insulator layer inward of the wafer. Then thethin-film IC is applied and bonded to the card substrate (step 104), andthe support substrate is removed, finally followed by connecting thethin-film IC and a wiring terminal on the card substrate by printing(step 105).

As described above, an LSI based on a silicon-on-insulator wafer isattached on a card substrate in a very thin form with a highreproducibility over a wide range by etching with the inner siliconoxide film as a boundary layer, and thus can be wired to componentsexternal to the LSI by printing.

FIG. 12 is a sectional view showing the condition immediately afterbonding a thin-film IC with a support substrate to a card substrate.Under this condition, the thin-film IC 16 is bonded to a transparentsupport substrate 18 by an adhesive 19 separable under ultraviolet rayon the one hand and to a card substrate 15 by a rubber-like adhesive 17on the other hand. The adhesive separable under ultraviolet ray is ofacrylic resin having an adhesive property under normal conditions andcontains a gel substance which hardens under ultraviolet ray thereby toexhibit separability. This adhesive is effective in separating thesupport substrate at room temperature with high reliability. Also, therubber-like adhesive 17 is capable of absorbing the stress caused whenthe card substrate is bent thereby to relax the stress concentration onthe thin-film IC. Further, the stress concentration due to thedifference in thermal expansion coefficient between the thin-film IC andthe card substrate can be relaxed. Therefore, the thin-film IC can bebonded to the card substrate with high reliability.

FIG. 13 is a sectional view showing the configuration of FIG. 12 fromwhich the support substrate 18 is removed. The support substrate 18 isremoved as it is not required once the thin-film IC 16 (designated by 20in FIG. 13) is bonded to the card substrate 15 (designated by 21 in FIG.13). Manual removal of the support substrate is facilitated by the useof an adhesive separable under ultraviolet ray which strongly bonds thethin-film IC to the support substrate 18 until radiation of theultraviolet ray. In this way, a very thin IC card substrate can bebonded with high stability.

FIG. 14 is a sectional view showing a configuration in which thethin-film IC 22 is bonded to the card substrate 25 and then thethin-wire IC is wired to the card substrate 25. The thin-film IC 22 hasa thickness of about 5 to 10 μm, and the adhesive for bonding thethin-film IC to the card substrate 25 a thickness of about 20 to 30 μm.The unevenness between the upper surface of the card substrate 25 andthe upper surface of the thin-film IC 22, therefore, is very small ascompared with the conventional case in which a bulk IC is bonded to thecard substrate. As a result, the thin-film IC and the wiring 23 on thecard substrate can be connected to each other by the wiring 24 ofconductive ink using the conventional printing technique. A great amountof connections is thus made possible within a short period of time. Massproduction and cost reduction of IC cards are thus realized.

FIG. 15 shows an example in which the configuration of the invention isused for packaging a multichip substrate. A thin-film IC 26 prepared bythe steps described above is bonded to a multichip substrate 27, andthen conductive ink 29 is connected to a wiring 28 on the multichipsubstrate 27 by printing. A low-cost multichip module thus can beobtained.

The above-mentioned conductive ink may be of any liquid material.

A printing unit used for fabricating a semiconductor device (IC card)according to the invention is schematically shown in FIG. 16. The ICcard according to the invention has the feature that a great amount ofconnections is made possible within a short length of time between theIC and the card substrate. The ink 32 in a wiring pattern is transferredto a rotary drum 31 and further to a substrate (before printing) 30carrying a thin-film IC thereon on a belt 34 passing along the side ofthe rotary drum rotating in high speed, thereby realizing a devicedelivered as a substrate (after printing) 33 carrying a thin-film IC.

FIG. 17 is a sectional view showing an example of the thin-film ICembedded in an IC card. The thin-film IC 35 is placed in a neutral plane37 of the card substrate 36 in such a manner as to resist the bend ofthe card substrate 36. Although when the card substrate is bent, atensile or compressive stress is exerted on the front and reverse sides,a thin-film IC located in a neutral plane 37 is not subjected to such aforce, thereby leading to a high bending strength and high reliability.

The industrial productivity and the reliability of the card are improvedby locating the LSI within about ±5% of one half of the IC cardthickness from the ideal neutral plane.

FIG. 18 is a diagram for explaining the steps for producing theconfiguration shown in FIG. 17. First, a thin-film IC 35 is adhered to acard substrate 36" and then a card substrate 36' of the same thicknessas the card substrate 36' is attached to the thin-film IC 35. As seenfrom the configuration shown in FIG. 17, the thin-film IC can thus beeasily embedded in the neutral plane of the IC card. A plurality of suchthin-film cards IC can be located at arbitrary positions of the cardsubstrate.

An embodiment of the invention is shown in FIG. 19. In the sectionalview of FIG. 19, the IC card is curved under the bending stress. Thethin-film LSI chip 104 is located on the center line 102a, i.e., theneutral plane of the card section, and therefore is highly resistive tothe bending force. In other words, no stress is exerted on the thin-filmLSI chip. Although the thin-film LSI chip is bent with the IC card, thesmall thinness of the thin-film LSI chip reduces the stress.

FIG. 20 is a diagram showing the LSI chip 105 in bent state. Character Rin FIG. 20 designates the radius of curvature from the center ofcurvature 107 to the center line 106 along the thickness of the LSI chip105, and character t the thickness one half of the LSI. Navier's theoremshows that the stress a in LSI surface is represented by E*t/R, where Eis may be considered Young's modulus of the LSI. Since the LSI surfaceis of silicon oxide, E is equivalently Young's modulus of silicon oxide.This equation indicates that the stress in the LSI surface isproportional to the thickness of the LSI and inversely proportional tothe radius of curvature. The LSI is considered to break due to bend whenthe surface stress exceeds the mechanical strength of the LSI. Thesurface stress is zero in the absence of bend since R is infinitelylarge. With the progress of bend and the decrease in R, the stress isprogressively increased until finally the LSI is broken. As the LSIthickness decreases with respect to the bend of the same radius ofcurvature, the surface stress decreases. An LSI sufficiently resistiveto bend, therefore, is realized by reducing the thickness to such adegree as not to reach the limit of mechanical breakdown. A thin LSIwhich stands alone, however, is difficult to handle. The handling isfacilitated and strength of a thin LSI increased by holding it betweenplastics or metal materials. In the process, it is necessary to positionthe thin-film LSI chip in the neutral plane of the material used forholding. In the case of an IC card, for example, the thin-film LSI chipis required to be located in the neutral plane as viewed from thesection of the card substrate as shown in FIG. 19. In this way, theneutral plane of the LSI coincides with the plane where stress exertedis zero when the card is bent. Even if the card is bent, the same effectis expected as if the thin-film LSI chip alone is bent.

Now, a method for forming a card using a thin LSI will be explained withreference to embodiments shown in FIGS. 21A, 21B and FIGS. 22A, 22B.First, as shown in FIG. 21A, a metallization pattern 109 is formed on alower card substrate 108.

The metallization pattern 109 is formed using conductive paste or ink oretching of a copper thin film. Under this condition, as shown in FIG.21B, a thin-film chip 110 is applied. An ordinary adhesive may be usedfor this purpose. In FIG. 22A, the thin-film chip 110 is connected byconductive paste 111, after which an upper card substrate 112 is bondedas shown in FIG. 22B. In the process, the lower card substrate 8 and theupper card substrate 12 are required to have the same thickness. In thisway, the thin-film LSI chip is located in the neutral plane of thecompleted card and therefore has a high resistance to bending stress. Ascompared with the conventional card, the card according to thisembodiment can be fabricated by integrating the card substrate with theLSI. Also, the connection using a conductive paste eliminates the needof wire bonding and makes possible fabrication of a low-cost, thin ICcard resistive to bend.

A top plan view of an IC card according to the invention is shown inFIG. 23. A thin-film LSI chip 114 and a conductor pattern 115 are formedon a flat IC card surface 113. A coil is shown as an example ofconductor pattern. This coil has the function of generating an inductiveelectromotive force in response to an electromagnetic wave received fromthe environment external to the IC card and supplying energy to thethin-film LSI chip. This coil pattern is connected to the thin-film LSIchip by the conductive paste. This coil has also the function ofreceiving data from outside of the IC card and delivering it to thethin-film LSI chip and sending the data from the thin-film LSI chip asan electromagnetic wave out of the IC card. The thin-film LSI chip, ifsituated at a corner of the card smaller in bending moment than at thecenter as viewed from the flat card surface, can be reduced in radius ofcurvature and an IC card is realized with a higher strength to thebending force. This method permits fabrication of an IC card ofnon-contact type which is high in reliability. The conventional IC cardclassified as a contact type has an electrode in the surface thereof,and therefore has the disadvantages of developing a contact failure oreasily succumbing to static electricity. The configuration according tothe invention, however, is applicable also to the conventional IC cardof contact type. FIG. 24 is a diagram showing an IC card having aconfiguration in which a thin-film LSI 16 is bonded in such a positionas to be surrounded by a flexible adhesive 119 like silicone. Thisconfiguration enables the adhesive 119 to cover the thin-film LSI chipwith a soft rubber-like material by taking advantage of the function ofbonding the upper card substrate 117 and the lower card substrate 118.Stress is thus made difficult to be exerted on the surface of the LSI,while at the same time making it possible to fabricate an IC cardresistive to the bending force. Further, even when the card substrate isdeformed by a very local impact force, the adhesive layer exhibits thefunction of relaxing the impact force, thereby preventing stress frombeing applied to the thin-film LSI chip.

FIG. 25 is a diagram showing the stress exerted on the LSI surface withthe card thickness as a parameter. The thin-film LSI is placed in theneutral plane of the card substrate, and the ratio is taken between thethickness of the LSI and that of the card to determine the stressexerted on the surface of the thin-film LSI. The stress in the LSIsurface is considerably related to the degree of card curvature. Thedegree to which the card is bent depends to a large measure on thethickness or material of the card, the force applied to the card and thecard position, and cannot be uniquely determined. By way of explanation,the position of the LSI is considered at the center in the neutral planeof the card, and the material of the card is deemed vinyl chloridecommonly used for magnetic and IC cards. The PET (polyethyleneterephthalate which is a heat-resistant crystalline thermoplastic)material is harder and more difficult to bend. The case involving vinylchloride, therefore, is considered applicable to considerably widepurposes. The radius of curvature which determines the bend depends onthe bending moment applied to the card. Such a moment is assumed to beapplied to the limit of bending the card. A simple actual measurementshows that a vinyl chloride card 0.76 mm thick has a radius of curvatureof 50 mm at the center thereof. Suppose the LSI has the same thicknessas the card. The stress in the LSI surface is given as 8E10*0.38/50 (Pa)from the stress equation, which is calculated as 600 MPa. Taking intoaccount the fact that LSI surface is mainly composed of a silicon oxidelayer, the property of glass can be safely assumed. The value for glasswas used as Young's modulus based on the standard table of physicalconstants.

The relation between radius of curvature and card thickness is affectedby the moment of inertia applied to the card. The radius of curvature Ris given as E*I/M, where E is Young's modulus of the card, I the momentof inertia and M the bending moment. The moment of inertia of the cardis proportional to the cube of card thickness. As a result, thecharacteristic curve of radius of curvature as shown in FIG. 26 isobtained.

From this characteristic, the stress in the LSI surface is determinedfor the ratio of 1.0 between the thickness of LSI and that of the card.As in the equation previously noted, the stress in the LSI surface asshown in FIG. 26 is determined. More specifically, the stress figure is2.4 GPa for the card thickness of 0.5 mm, and 5.4 GPa for the thicknessof 0.25 mm. Under this condition, the LSI tends to easily break.Actually, therefore, the LSI is formed in thin film and held in theneutral plane of the card. FIG. 25 is a diagram showing the stress inthe surface of the thin LSI plotted against the thickness ratio betweenLSI and card as a parameter. This graph is shown in enlarged form inFIG. 27 with the thickness ratio between LSI and card expanded in therange of 0 to 0.16. The region where the LSI can stand the bend isassumed to be 90 MPa and the same as the breaking strength of glassbased on the standard table of physical constants. As a result, thethickness required of the thin-film LSI can be determined for each cardthickness, and hence the limit to which the LSI thickness can bereduced. In other words, the required LSI thickness is 110 microns orless for the card thickness of 0.76 mm, 19 microns or less for the cardthickness of 0.5 mm, and 4 microns or less for the card thickness of0.25 mm. The reliability of course can be greatly improved by reducingthe LSI thickness to the limit.

As described above, a semiconductor device configured according to thisinvention solves the problems of the prior art, and provides a reliable,low-cost IC card or a multichip module. In other words, a thinsemiconductor element with a protective insulating layer covered on thereverse side thereof according to the method described above preventsintrusion of ionic contaminants by way of the reverse side of thesemiconductor element nearest to the external environment for animproved reliability. As a consequence, an IC card with an improveddurability can be fabricated by bonding a thin LSI to a substrate usingan inexpensive organic adhesive usually containing a considerable amountof ionic impurities.

If silicon nitride is used as the abovementioned protective insulatinglayer which is high in thermal expansion coefficient, the curling of thethin-film LSI due to the internal residual stress can be suppressed,thereby contributing to an improved reliability of the IC card.

On the other hand, the use of a SOI wafer permits the inner insulatorlayer to function as a stopper layer in fabrication processes andtherefore a very thin IC can be fabricated with a superiorreproducibility. The thin-film IC has a thickness of 5 to 10 μm. Such athin IC as this is resistive to bend, and when coupled to a thinsubstrate like an IC card by flexible adhesive, is further improved inbend resistance for an improved reliability.

Also, a thin-film IC, which by itself is fragile, can be fabricated inhighly stable form by being mounted on a support substrate in advance.The support substrate to which the IC is coupled can be reliably removedat low temperatures if an adhesive separable under ultraviolet ray isused for coupling. The thin-film IC attached on the card is so thin thatthe substrate and IC can be wired to each other by printing ink, therebymaking it possible to fabricate an IC card which is low in cost and flatin shape.

The methods described above are applicable not only to IC cards but alsoto the packaging of ICs of similar types and multichip packaging aswell.

Taking a sectional view of a flat IC card in bent condition, a curvedsurface develops an elongation and the reverse side thereof acompression. Under this condition, the central portion of the particularsection of the IC card is subjected to less stress and free ofcompression. A thin IC chip present at this portion is subjected tolesser stress. The thinner the IC chip, the better. When the card isthick, however, the IC chip can be increased in thickness to some degreesince the increased critical curvature of the card makes it difficult tobend due to card rigidity. When the IC card is thin, by contrast, thethickness of the IC chip must be reduced in order to facilitate bendingand hence to relax the stress in the IC chip. In reducing the thicknessof the IC, the thinner the LSI, the higher accuracy is required of thefabrication equipment. Changing the required thickness of the IC chipaccording to the thickness of the IC card is very important from theeconomic viewpoint and also for securing the reliability. In view ofthis correlation present between the thickness of the IC card and thatof the IC chip, the proper thickness of the IC chip is considered 110microns or less for the thickness of a completed IC card of 760 micronsor more, 19 microns or less for the thickness of a completed IC card of500 microns or more, and 4 microns or less for the completed IC cardthickness of 250 microns or more. Thus an economical and highly reliableIC card is produced.

What is claimed is:
 1. An IC card comprising:an IC chip having athickness of 110 microns or less; a conductive pattern including a coilfor supplying energy to said IC chip; and first and second flexiblesubstrates having said IC chip and said conductive pattern interposedtherebetween; wherein said IC chip is located so that one surface ofsaid IC chip is subjected to compressive stress and an opposite surfaceof said IC chip is subjected to tensile stress when said IC card isbent.
 2. An IC card according to claim 1, wherein said IC chip and saidconductive pattern are connected with a conductive paste.
 3. An IC cardaccording to claim 1, wherein said first substrate is vinyl chloride orPET.
 4. An IC card according to claim 1, wherein said IC chip isdisposed on a plane located at one-half the thickness of the IC card. 5.An IC card according to claim 1, wherein a plurality of additional ICchips are provided on said first substrate.
 6. An IC card according toclaim 1, wherein said one surface and said opposite surface are each incontact with the flexible substrates.
 7. An IC card according to claim1, wherein said IC chip is interposed between metallic films.
 8. An ICcard according to claim 6, wherein said one surface and said oppositesurface are each in contact with the flexible substrates via anadhesive.
 9. An IC card comprising:a conductive pattern including a coilfor inputting information to an IC chip and for outputting informationfrom said IC chip; and first and second flexible substrates having saidIC chip and said conductive pattern interposed therebetween; whereinsaid IC chip is located so that one surface of said IC chip is subjectedto compressive stress and an opposite surface of said IC chip issubjected to tensile stress when said IC card is bent, and wherein saidIC chip has a thickness of 110 microns or less.
 10. An IC card accordingto claim 9, wherein said one surface and said opposite surface are eachin contact with the flexible substrates.
 11. An IC card according toclaim 9, wherein said IC chip is interposed between metallic films. 12.An IC card according to claim 10, wherein said one surface and saidopposite surface are each in contact with the flexible substrates via anadhesive.
 13. An IC card comprising:a first substrate formed with aconducting pattern including a coil for supplying energy and inputtinginformation to an IC chip and for outputting information from said ICchip; said IC chip which is disposed on said first substrate, connectedto said conductive pattern, and which has a thickness of 110 microns orless; and a second substrate provided on said first substrate to coversaid IC chip, wherein said IC chip is located so that one surface ofsaid IC chip is subjected to compressive stress and an opposite surfaceof said IC chip is subjected to tensile stress when said IC card isbent.
 14. An IC card according to claim 13, wherein said IC chip isinterposed between metallic films.
 15. An IC card comprising:an IC chiphaving a thickness of 110 microns or less; and first and second flexiblesubstrates having said IC chip interposed therebetween, wherein said ICchip is thinner than said first and second substrates, and wherein saidIC chip is disposed in a position where one surface of said IC chip issubjected to compression and an opposite surface is subjected to tensionwhen said IC card is bent.
 16. An IC card comprising:a first substrateon which a wiring is formed; a thin film IC chip disposed on said firstsubstrate, connected to said wiring and having a thickness of 110microns or less; and a second substrate provided on said first substrateto cover said IC chip; wherein said IC chip is thinner than said secondsubstrate, and wherein said IC chip is located so that one surface ofsaid IC chip is subjected to compressive stress and an opposite surfaceof said IC chip is subjected to tensile stress when said IC card isbent.
 17. An IC card comprising:a first substrate on which a wiring isformed; a thin film IC chip disposed on said first substrate, connectedto said wiring and having a thickness of 110 microns or less; and asecond substrate disposed on said first substrate to cover said IC chip;wherein a thickness of said IC chip is smaller than a distance from asurface of said IC chip to a surface of said IC card, and wherein saidIC chip is located so that one surface of said IC chip is subjected tocompressive stress and an opposite surface of said IC chip is subjectedto tensile stress when said IC card is bent.
 18. An IC cardcomprising:first and second substrates; and an IC chip disposed betweensaid first and second substrates, connected to a wiring disposed betweensaid first and second substrates, and having a thickness of 110 micronsor less, wherein said IC chip is disposed so that one surface of said ICchip is subjected to compressive stress and an opposite surface issubjected to tensile stress when said IC card is bent.
 19. An IC cardaccording to claim 18, wherein said IC chip is interposed betweenmetallic films.
 20. An IC card comprising:an IC chip having a thicknessof 5 to 10 microns; a wiring connected to said IC chip; and first andsecond flexible substrates having said IC chip and said wiringinterposed therebetween, wherein said IC chip is located so that onesurface of said IC chip is subjected to compressive stress and anopposite surface of said IC chip is subjected to tensile stress whensaid IC card is bent.
 21. An IC card according to claim 20, wherein saidIC chip is adhered to said first substrate by rubber-like adhesive. 22.An IC card according to claim 20, wherein said IC chip is connected tosaid wiring by liquid conductive material.
 23. An IC card comprising:aflexible upper substrate; a flexible lower substrate on which a wiringis formed; and an IC chip disposed between said upper and lowersubstrates and having a thickness equal to or less than 110 microns,connected with said wiring by a conductive adhesive, wherein said ICchip is located so that one surface of said IC chip is subjected tocompressive stress and an opposite surface is subjected to tensilestress when said IC card is bent.
 24. An IC card according to claim 23,wherein said IC chip is interposed between metallic films.
 25. An ICcard comprising:an IC chip having a thickness of 110 microns or less; aconductive pattern including a coil for supplying energy to said ICchip; and first and second flexible substrates having said IC chip andsaid conductive pattern interposed therebetween, wherein said IC chip islocated so that one surface of said IC chip is subjected to compressivestress and an opposite surface of said IC chip is subjected to tensilestress when said IC chip is bent, and wherein said IC chip isstrengthened by metal or plastic material.
 26. An IC card comprising:aconductive pattern including a coil for inputting information to an ICchip and for outputting information from said IC chip; and first andsecond flexible substrates having said IC chip and said conductivepattern interposed therebetween, wherein said IC chip is located so thatone surface of said IC chip is subjected to compressive stress and anopposite surface of said IC chip is subjected to tensile stress whensaid IC card is bent, and wherein said IC chip is strengthened by metalor plastic material, and wherein said IC chip has a thickness of 110microns or less.
 27. An IC card comprising:a first substrate formed witha conducting pattern including a coil for supplying energy and inputtinginformation to an IC chip and for outputting information from said ICchip; said IC chip, which is disposed on said first substrate, connectedto said conductive pattern, and has a thickness of 110 microns or less;and a second substrate provided on said first substrate to cover said ICchip, wherein said IC chip is located so that one surface of said ICchip is subjected to compressive stress and an opposite surface of saidIC chip is subjected to tensile stress when said IC card is bent, andwherein said IC chip is strengthened by metal or plastic material. 28.An IC card comprising:first and second substrates; and an IC chipdisposed between said first and second substrates, connected to a wiringdisposed between said first and second substrates, and having athickness of 110 microns or less, wherein said IC chip is disposed sothat one surface of said IC chip is subjected to compressive stress andan opposite surface is subjected to tensile stress when said IC card isbent, and wherein said IC chip is strengthened by metal or plasticmaterial.
 29. An IC card comprising:a flexible upper substrate; aflexible lower substrate on which a wiring is formed; and an IC chipdisposed between said upper and lower substrates and having a thicknessequal to or less than 110 microns, connected with said wiring by aconductive adhesive, wherein said IC chip is located so that one surfaceof said IC chip is subjected to compressive stress and an oppositesurface is subjected to tensile stress when said IC card is bent, andwherein said IC chip is strengthened by metal or plastic material. 30.An IC card in which a flexible IC chip and a conductive pattern areinterposed between flexible substrates in a location to be subjected tobending when the IC card is bent, wherein:a thickness of said IC chip isequal to or less than 110 microns; said IC chip is strengthened by metalor plastic material; an external terminal of said IC chip and saidconductive pattern are electrically connected by a conductive adhesive;and said IC chip is located so that one surface of said IC chip issubjected to compressive stress and an opposite surface of said IC chipis subjected to tensile stress when said IC card is bent.
 31. An IC cardaccording to claim 30, wherein said conductive pattern includes a coil.32. An IC card according to claim 30, wherein said coil supplies energyto said IC chip.
 33. An IC card according to claim 30, wherein said coilinputs information to said IC chip and outputs information from said ICchip.
 34. An IC card according to claim 30, wherein said IC chip is heldbetween at least two flexible substrates.